Proof first

An old lady goes into a supermarket and takes 3 tins of cat food and walks up to the checkout to pay.

"No, I can't sell them to you," said the woman at the checkout.

"Why not?" asked the old lady.

"We need proof that you own a cat. You see, some old people are known to take cat food home and eat it for themselves, according to government research," explained the woman, "and the government forbid us to sell unless you have proof."

"Hold that thought," said the old lady. She rushes home and comes back holding her cat, and the woman sells her the cat food.

The next day the same old lady comes in and tries to buy 3 tins of dog food.

"No, I can't do that," said the woman. "We need proof that you own a dog, because government research says.."

"Whatever," snapped the old lady. She rushes home and brings her dog in and takes the dog food.

The next day the old lady comes in, holding a large tank with a hole on the top.

"Stick your hand in there," said the lady.

"There could be something poisonous in there like a python," said the woman at the checkout.

After the old lady finally persuades her that there is nothing dangerous in there, the woman sticks her hand in.

"This feels like shit," said the woman.

"There's your proof. Can I have three rolls of toilet paper?" asked the old lady.